Ab initio study of the structure and dynamics of solvated highly charged metal ions

The influence of highly charged cations on the structure and dynamics of the aqueous phase is investigated by performing Ab Initio Molecular Dynamics (AIMD) simulations on the Fe³⁺ and Ca²⁺ cations and the CaCl₂, FeOH²⁺ and AlOH²⁺ species all solvated by up to 64 waters. A detailed comparison between results of Fe³⁺ and a previous study of the Al³⁺ ion reveal significant changes in the hydrogen bond structure of 1st and 2nd hydration shells between the two systems. Differences are also noticed in the dynamics of the 2nd hydration shell. An orbital interaction is observed between Fe³⁺ and water that is not observed for Al³⁺. The results of the Ca²⁺ AIMD simulation show a more tetrahedral H-bonding structure relative to 3+ cations. Solvent exchange between the coordinating waters of Ca²⁺ and bulk proceeds via the associative interchange mechanism. Ambient and high temperature (near critical, 650K) simulations of the CaCl₂ system show that the Ca²⁺+ and Cl- ions exist as solvent separated ion pairs under ambient conditions while at 650K the Ca-Cl contact pairs are formed. This is accompanied by a significant disruption of the hydrogen bond structure of the solvent. The [FeOH]²⁺ and [AlOH]²⁺ aqueous species are found to display a significant labilizing effect due to the presence of the hydroxide species in the 1st hydration shell. A uniform destabilizing of all 1st shell waters is observed for the AlOH²⁺ species while for the FeOH²⁺ species some hydrating waters feel a stronger effect determined by their position relative to the OH⁻ specie

Ab initio study of the structure and dynamics of solvated highly charged metal ions

The influence of highly charged cations on the structure and dynamics of the aqueous phase is investigated by performing Ab Initio Molecular Dynamics (AIMD) simulations on the Fe³⁺ and Ca²⁺ cations and the CaCl₂, FeOH²⁺ and AlOH²⁺ species all solvated by up to 64 waters. A detailed comparison between results of Fe³⁺ and a previous study of the Al³⁺ ion reveal significant changes in the hydrogen bond structure of 1st and 2nd hydration shells between the two systems. Differences are also noticed in the dynamics of the 2nd hydration shell. An orbital interaction is observed between Fe³⁺ and water that is not observed for Al³⁺. The results of the Ca²⁺ AIMD simulation show a more tetrahedral H-bonding structure relative to 3+ cations. Solvent exchange between the coordinating waters of Ca²⁺ and bulk proceeds via the associative interchange mechanism. Ambient and high temperature (near critical, 650K) simulations of the CaCl₂ system show that the Ca²⁺+ and Cl- ions exist as solvent separated ion pairs under ambient conditions while at 650K the Ca-Cl contact pairs are formed. This is accompanied by a significant disruption of the hydrogen bond structure of the solvent. The [FeOH]²⁺ and [AlOH]²⁺ aqueous species are found to display a significant labilizing effect due to the presence of the hydroxide species in the 1st hydration shell. A uniform destabilizing of all 1st shell waters is observed for the AlOH²⁺ species while for the FeOH²⁺ species some hydrating waters feel a stronger effect determined by their position relative to the OH⁻ specie

Aqueous Al3+ speciation from Density Functional Theory calculations

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Influence of F - coordination on Al 3+ hydrolysis reactions from density functional theory calculations

Aqua/hydroxo mononuclear Al 3+ species coordinated by F - in aqueous solution are investigated using density functional theory (DFT B3LYP/6-311++G(d,p)) and the polarized continuum model (PCM). Optimized gas-phase geometries have been obtained for the species AlF(OH) n(H 2O) m(2-n) + in which n = 0, 1, 2, or 3 while (n + m) = 3, 4, or 5. Analysis of the Al-F, Al-O, and O-H bond lengths and the Al, F, O, and H natural charges of these complexes reveals clear trends that suggest increased acidity with decreasing coordination number (CN) and decreased water stability with increased hydrolysis. These observations are supported by the calculation and analysis of the dehydration and hydrolysis reaction Gibbs free energies δGaqueous dehydration and δGaqueous hydrolysis of the AlF(OH) n(H 2O) m(2-n) + complexes, which clearly show a strong correlation between increased hydrolysis and a preference to coordinate fewer water molecules. The combination of the appropriate δGaqueous dehydration and δGaqueous hydrolysis values generate the aqueous Gibbs free energies relative to AlF(H 2O) 5 2+ and demonstrate the clear transition from a 6 to 5 to 4 coordinate species as a function of ligand hydrolysis. Calculation of the equilibrium mole fraction of each species as a function of pH shows that this system is largely dominated by the AlF(OH) 1(H 2O) 4 1+ and AlF(OH) 3 1- species. A comparison of structural and electronic data with the aqueous Al 3+ complexes shows a remarkable similarity when plotted against the number negative ligands (F - or OH -), suggesting that the F - anion coordinates the Al 3+ cation in a similar way to the remaining OH - anions. The comparison of the calculated equilibrium mole fractions of each species displays important changes in the composition of our model system upon Al 3+ coordination by F - in the direction of increased acidity of these complexes. Our predicted decreased stability of the Al-water bond is in complete agreement with experimental NMR observations of an increased water exchange rate upon F - coordination of aqueous aluminum complexes. Our prediction of stable hydroxide ternary complexes is not in agreement with recent NMR data, which indicate that these complexes do not readily form. An explanation for this may lie in the increased lability of these complexes, which may lead to difficulties in NMR detection. © 2011 American Chemical Society.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Rydberg electron capture by neutral Al hydrolysis products

We predict that electron attachment may be used with ESI-MS techniques to observe neutral Al metal aqua-oxo-hydroxo species and the complex polymerization and precipitation reactions in which they participate. Neutral aqueous metal species have, so far, been invisible to ESI-MS techniques. This journal is © 2013 the Owner Societies.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Modeling the aqueous Al3+ system using Density Functional Theory

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Improved DFT-based interpretation of ESI-MS of aqueous metal cations

We present results showing that our recently developed density functional theory (DFT)-based speciation model of the aqueous Al3+ system has the potential to improve the interpretations of ESI-MS studies of aqueous metal cation hydrolytic speciation. The main advantages of our method are that (1) it allows for the calculation of the relative abundance of a given species which may be directly assigned to the signal intensity in a mass spectrum; (2) in cases where species with identical m/z ratios may coexist, the assignment can be unambiguously assigned based on their theoretical relative abundances. As a demonstration of its application, we study four pairs of monomer and dimer aqueous Al3+ species, each with identical m/z ratio. For some of these pairs our method predicts that the dominant species changes from the monomer to the dimer species under varying pH conditions. [Figure not available: see fulltext.] © 2013 American Society for Mass Spectrometry.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

The influence of F- coordination on Al3+ hydrolysis reactions from Density Functional Theory

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Ab initio molecular dynamics simulations of the hydration shells of highly charged metal ions in aqueous solutions

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Ion association in AlCl3 aqueous solutions from Constrained First Principles Molecular Dynamics

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